The present invention relates generally to radio telecommunication systems and, more specifically, to multi-cell radio telecommunication systems for communication between two or more telecommunication units, at least one of which can be mobile.
In recent years various techniques and concepts of private and public radio telecommunication systems for telephony and data transmission have been developed. In general two types of radio telecommunication systems can be distinguished.
The first is cellular communication, providing coverage over a relatively wide area, i.e. relatively large cells. Analogue cellular systems, such as designated AMPS, ETACS, NMT-450 and NMT-900 have been deployed throughout the world. Digital cellular systems are designated IS-54B in North, America and the pan-European GSM system.
The second is cordless communication, ranging from simple single-cell residential cordless telephones to multi-cell business cordless systems capable of serving hundreds or even; thousands of cordless communication units across (large) offices, production halls etc. Analogue cordless telephones are designated CT0, CT1 and CT1+. Amongst the digital cordless systems, designated CT2, CT2-CAI, CT3 and DECT (Digital. Enhanced Cordless Telecommunications), both CT3 and DECT use TDMA (Time Division Multiple Access) as their transmission technique, whereas CT2 operates under FDMA (Frequency Division Multiple Access). Besides these access techniques, in particular in North America, spread spectrum access is used for cordless communication. CDMA (Code Division Multiple Access) is another digital access technique which can be used for cordless communication.
Some of the cordless technologies can also be used to provide a type of Personal Communication Services (PCS) or also called Cordless Terminal Mobility (CTM). This is basically a multi-cell radio telephone like service that covers a whole town or city. Customers of this service are able to carry their mobile telecommunication units (telephone handsets) with them and can make calls in the coverage area of the service.
Within the public domain, there is a growing interest in replacing the wired local loop of the conventional Public Switch Telephone Networks (PSTN) and also the Integrated Services Digital Networks (ISDN) wired telephone systems by a so-called wireless local loop, i.e. using radio technology as an alternative for or as a replacement of the copper wires down to the subscriber premises. Within this concept of Radio in the Local Loop (RLL) two basic systems can be distinguished: Fixed RLL (FRLL) and Mobile RLL (MRLL). In the FRLL system, the subscriber is provided with an ordinary telephone socket, however connected to a radio transceiver, also called Fixed Access Unit (FAU) or Wireless Fixed Access Unit (WFAU). Via this FAU/WFAU a radio link is established with a so-called radio access unit, which provides access to the PSTN/ISDN. In the MRLL concept, the subscriber is provided with a portable cordless or mobile radio telephone handset, by which, via the radio access unit, direct access to the PSTN/ISDN can be established.
Mixed concepts are also possible, i.e. FRLL providing mobility in the subscriber premises, also called Cordless :In The Home (CITH) and residential or neighbourhood mobility, also called Cordless In The Neighbourhood (CITN). In some countries, governmental regulations prevent the established telephone operators from offering local mobility in the present PSTN/ISDN. In such cases, it is very advantageous for a second or third operator to offer both fixed and mobile or cordless access to the PSTN/ISDN. The radio access units of an RLL system are arranged to provide a local multi-cell radio telecommunication environment.
Conventional multi-cell radio telecommunication systems consist typically of a plurality of radio access units generally called base stations (BS) each providing service to a geographical area or cell, and central control units, also called radio exchanges (RE) or mobile telephone switching offices (MTSO), to which a number of base stations are connected. The MTSO and/or RE are in turn coupled to a PSTN/ISDN for completing transmissions, such as telephone calls, between cordless or mobile radio subscribers and landline subscribers.
In a multi-cell radio telecommunication system the total capacity of the system is specified in terms of the area covered by all the base stations together and the communication or call/ data processing capabilities of the combination of the central control unit and the base stations.
From a system functionalities point of view, the multi-cell radio system is comprised of a central processing functionality implemented in the central control unit and distributed functionalities in the several base stations. Interface functions are required both at the central control unit and the base stations to support the connection between the central control unit and the base stations.
A multi-cell cordless radio system of this type is disclosed by EP-A-0 637 144. The base stations, called Radio Fixed Part (RFP), connect to a battery of transcoders (TRAS) which transcoders connect to the PSTN through a Central Control Fixed Part (CCFP), via link lines L by means of Drop/Insert (DI) means.
In order to extend the range of a base station or to improve the link quality between a base station and a remote radio telecommunication terminal, called Portable Part (PP), repeater devices are disclosed, indicated by RIP. The repeater devices are operative at the radio air interface between a base station and a remote radio telecommunication terminal, using the same type of radio air interface and communication protocol, on the one hand, between the repeater device and the radio base station and, on the other hand, between the repeater device and the remote radio telecommunication terminal. The type of radio air interface and communication protocol disclosed is DECT.
In the known multi-cell radio telecommunication systems, the base stations are connected to the central control unit, i.e. the RE or MTSO, via copper wire cables, optical fibre cables or special purpose radio links, using proprietary or standardised interfaces and communication protocols, such as is the case of the pan-European GSM system.
For the air interface or radio link between the base stations and the remote radio telecommunication terminals such as the radio telephone handsets, interface protocols are used which are defined by the particular radio system such as DECT or GSM, for example.